Human serum ultrafiltrate (UFS) obtained from cancer patients contains material(s) which inhibit the cytolytic activity of tumor necrosis factor (TNF-alpha) and lymphotoxin (LT-TNF-beta) in vitro. These factors are found in UFS from patients with different types of cancers and are not detected in the sera of normal donors. Results from molecular sizing studies demonstrated that their molecular weights are over 30 kD. They appear to block by interfering with the early stages of cell lysis. It is not clear whether or not these TNF-LT blocking factor(s) are the same molecules. These factors may have adverse effects on the biological activities of TNF and LT in cancer patients.